1. Field of the Invention
This invention relates to an oil type release agent for metal casting which is designed to be used for the casting of non-ferrous metals such as aluminum, magnesium, zinc, etc., to a spray method using the oil type release agent, and to an electrostatic spray apparatus.
2. Description of the Related Art
As is well known, not less than 99% of release agents that have been used past 40 years for the casting of non-ferrous metals such as aluminum, magnesium, zinc, etc. have been occupied by water-soluble release agents. Meanwhile, oil type release agents, which make it possible to perform the casting of non-ferrous metals applying spray as low as 1/500- 1/1000 times the amount required in the case of the water-soluble release agents, have begun to be used from three years ago. However, since the amount of oil type release agents to be sprayed is very little, the oil film to be adhered on a mold becomes insufficient sometimes, especially in the case of a complicated mold in configuration or a large mold in size. For example, in the case of the complicated mold in configuration, the formation of an oil film becomes especially insufficient at the portions of the mold which are hidden from the spraying direction. Additionally, since the mold is constituted by convex and concave portions, the release agent film tends to become thick at the concave portions.
Conversely, the release agent film tends to become thinner at the convex portions. Because of this, while the release agent is more likely to build up excessively at the concave portions, thereby giving rise to one of the causes for an increase of porosity, a soldering of a casting product may be caused to occur at the convex portions due to insufficiency of mold release film at the convex portions. In order to cope with this problem, currently, measures are taken to perform the casting by increasing the quantity of spray of release agent so as to enable the splashed particles thereof to sufficiently reach the hidden portions and the convex portions while sacrificing the increase in more or less of porosity. On the other hand, in the case of a large size mold, the thermal energy of a molten non-ferrous metal is large. Therefore, the temperature of the entire body of the mold, especially the temperature of thin-walled portions, is caused to rise approaching the temperature of the molten metal, thus increasing the temperature to 350° C. or more.
As a result, Leidenfrost's phenomenon may take place in an oil type release agent, thus allowing the droplets of release agent to boil. Further, due to this boiling, the droplets that splash from the mold onto the floor may be caused to increase. Because of this, the oil film to be formed would become thinner, thus possibly deteriorating the mold release characteristics. In order to cope with this problem, two different methods have been adopted. One of them is to spray a relatively large quantity of release agent, thereby increasing the thickness of the oil film. The other is to spray a small amount of water onto the thin-walled portions which become high temperatures to thereby cool the thin-walled portions, and then to spray an oil type release agent. When the release agent is sprayed in a relatively large quantity, the portions of mold where the oil film has already been formed to a sufficient thickness are also increased in thickness of the oil film. As a result, the quantity of porosity is more likely caused to increase in the cast product. Additionally, the strength of the cast product may sometimes be decreased slightly. In the latter method, conversely, even if the quantity of water to be applied is small, the installation of piping is required for spraying water.
As explained above, the conventional techniques are accompanied with the following problems. Namely, it is impossible to sufficiently feed a release agent to hidden portions of the mold, thereby making it impossible to form an appropriate oil film at such hidden portions. Further, it is impossible to form a uniform oil film at convex and concave portions, thereby necessitating the excessive spraying of release agent. Furthermore, it is impossible to form a satisfactory oil film at the thin-walled portions of the mold.
Under the circumstances, there have been conventionally proposed various kinds of modified techniques.
JP-A 6-182519 (KOKAI) (Patent Document 1) describes a technique wherein the droplets of release agent are negatively charged by making use of a spray apparatus and sprayed onto a mold which has been positively charged, thereby enabling the droplets of release agent to reach the hidden portions of mold. However, in the case of this technique, since the electrical conductivity of a water-soluble releasing agent is so high, it is impossible to decrease the electrical conductivity even if the quantity of water is reduced. Because of this, electrostatic spraying cannot be applied to any water-soluble releasing agents. Further, since the electric insulating properties of an oil type releasing agent are so high, electrostatic spraying cannot be applied to any oil type releasing agents either.
JP-A 2001-259787 (KOKAI) describes a technique similar to that of aforementioned Patent Document 1, and is directed to a technique using a water emulsion containing a large quantity of silicones. However, this technique cannot be applied to an oil type release agent.
JP-A 9-235496 (KOKAI) describes, as means for providing a paint with electrical conductivity, a technique for lowering the electric resistance through the use of alcohol or ammonium salt as an electrostatic assistant. JP-A 2000-153217 (KOKAI) describes a technique suggesting the addition of an electrostatic assistant to a paint. However, the “electrostatic assistant exhibiting strong chemical polarity” can be dissolved in the “oil type release agent exhibiting weak chemical polarity” at a ratio of only 0.3% by weight, thus generating sedimentation or separation. According to the studies made by the present applicant, it was found impossible to recognize the adhesion enhancing effects of the electrostatic assistant at such a low level. Although the solubility of the electrostatic assistant can be enhanced by the addition of polar solvents, the health of cast workers may be damaged by the addition of polar solvents. For this reason, i.e., taking the health of workers into consideration, no polar solvents are included in the composition of an oil type release agent.
Other conventionally known related arts include a technique wherein a high-temperature type release agent and a low-temperature type release agent are separately applied by means of electrostatic spraying as disclosed in JP-A 61-42462 (KOKAI), and a technique wherein an ultrasonic wave is employed, as disclosed in JP-A 61-182519 (KOKAI).